Examinations are generally conducted on through holes each having a diameter of several ten˜several hundred micron millimeters to check if a correct number of the through holes are opened, if any foreign matters are present in the through holes, and the like. Optical methods are generally conducted for such examinations. For example, an area sensor camera is used to photograph an examining surface of a work piece where through holes are formed, and an image processing apparatus is used to compare the result with a reference value to make a determination as to whether or not the result is good. However, in the conventional example described above, beside the image processing apparatus, an auto-focusing unit, a microscope, and electron beams are required, whereby the examination is conducted for each one of the through holes, or with an expanded image that is expanded at a high level of magnification.
In a general structure of this type of examination apparatus, as shown in FIG. 14, a work piece 1 having through holes is disposed at a fixed position, a light source 2 is disposed on a lower side thereof and an image taking device that integrates a microscope unit 3 and an area sensor camera 4 is disposed on an upper side thereof. The image taking device can be elevated or lowered by a Z-axis automatic control system 5, and the area sensor camera 4 is connected to an image processing apparatus 7 through an automatic focusing unit 6. The examination apparatus is used to photograph the work piece having a plurality of through holes 8 as shown in FIG. 15. An image is taken for every one of the through holes 8 as one unit, and a comparing process to compare a pixel area of a reference through hole and a pixel area of a measured through hole is conducted to determine as to whether the through hole is good or bad. For example, when a foreign matter is present in the through hole 8, the amount of the passing light is reduced, and therefore the measured pixel area becomes smaller, such that the through hole 8 having a pixel area that is below a specified threshold value is determined as being defective.
However, the conventional method described above has the following problems. When the number of through holes increases, it takes too long to conduct the examination. When a highly magnified image is to be taken, demands for mechanical precision of the apparatus become stricter, and therefore its manufacturing cost becomes higher. In other words, because the depth of focus becomes narrower, the automatic focusing unit 6, the automatic-control system for camera's z-axis 5, and the like are required. Also, because the range of field of view becomes narrower, the table on which the work piece 1 is mounted requires a high positioning accuracy.
Furthermore, when foreign matters in through holes are examined by the photographing method using the conventional area sensor camera, the number of pixels is limited and therefore its resolving power is limited. In particular, when foreign matters are light-transmissive, their recognition is extremely difficult, and therefore there are possibilities to erroneously detect good products as bad products.
The present invention focuses on the problems of the conventional art described above, and it is an object of the present invention to provide a method and an apparatus for examining foreign matters in through holes, which can quickly make determinations with low costs and high accuracy.